The present invention relates to an image processing system which has a function of inputting, storing or receiving, as a binary data, an image, in particular, an image such as a document including a photograph having a varying brightness density and of outputting a multi-valued, half-tone image to a display, a printer or the like.
In apparatuses such as facsimile machines and electronic filing systems for handling document data as image data, in general, an image is processed as binary data having white and black values. In this case, such an image having a varying brightness density as a photograph is subjected to a half-tone dot screening operation called "dither". There have been already widely known many dither methods which include, for example, an organization dither method and a mean error least method (e.g., refer to a paper Japanese article of Journal of The Institute of Image Electronics Engineers of Japan, Vol. 10, No. 5, pp. 388-397).
However, the dither image is remarkable in the shape of the half-tone dot screening at the time of its output. To avoid this, there have been studied several systems wherein the multi-valued image data is again subjected to a conversion at the time of its output to output a multi-valued half-tone image. One of its detailed systems have been suggested, for example, in a Japanese periodical "Technique Of Restoring Binary Image Into Half-Tone Image", Image Labo., March 1990, pp. 19-24.
However, in these systems, since dither images converted into binary forms by an organization dither method are handled, no consideration is paid to the application of these systems to such random dither images which have been recently increasingly utilized based on a mean error least method or the like.
Meanwhile, U.S. patent application Ser. No. 07/272,447 filed Nov. 17, 1988 (JP-A-2-90377) discloses a system in which a half-tone image is reproduced from a binary image independently of a dither method. However, since this system is intended to convert the reproduced image to a binary form again and then output the image, no consideration is paid to the quality of the reproduced image itself in this system.
When it is desired to reproduce a half-tone image from a dither image including a random dither image, the prior art systems have had problems which follow.
The processing of reproducing a multi-valued half-tone image from a binary dither image is a sort of filtering processing with use of a low pass filter. A system for counting the number of white pixels in a certain region and using the white pixel number counted as a brightness or luminance means that the filter is completely flat in its certain frequency range, which results in that a reproduced image is removed in its high frequency components and thus a resultant image display is blurred.
In a random dither image, the distribution density of black pixels macroscopically coincides with the average luminance of the region but its local distribution density varies from region to region.
Since many of systems based on the organization dither method assume that white and black pixel appearance frequency is periodical or cyclical, the random dither method is unsuitable for these systems.
The above-mentioned U.S. application Ser. No. 07/272,447 teaches a system in which brightness is calculated through the convolution operation of a dither image within a window and a Gaussian function with use of a filter having specific spatial frequency characteristics. In this system, however, it is difficult to determine a suitable filter coefficient. Further, no consideration is paid, at all, in this system to the case where a color image is handled. Furthermore, white and black pixels are present as isolated points in regions having very low and high brightness. For example, in the case where an original half-tone image has 256 stages of tones, only one of 256 pixels is a white pixel even on average in a region having a brightness of 1/256. When such an image is scanned with a very small reference window, all the pixels within the window are black pixels in most case, and one or more white pixels are present in the window in very few cases. When such an image is reproduced with a single filter, brightness is concentrated rated on a particular part of the region.
FIG. 2 shows an example. More specifically, when such a low brightness region as shown in FIG. 2A is subjected to a dither processing, white pixels are less generated as shown in FIG. 2B. When the dither image is subjected to a half-tone image reproducing operation with use of a filter as shown in FIG. 2D, an output image takes such values as shown in FIG. 2C. The output image has substantially the same average brightness as the region of FIG. 2A but the values of pixels of the output image are concentrated on certain pixels. Thus, this results in generation of drop-like noise in the image and thus deterioration of its image quality.